1. Field of the Invention
This invention relates to a thermal printer which effects printing by using a thermal head comprised of a line of print elements, and more particularly, to a thermal printer for issuing labels including bar codes.
2. Description of the Related Art
In recent years, many retail shops use labels which are affixed on articles such as foodstuffs and other daily sundries and which are indicative of information pertaining to articles, to simplify sales transactions. A typical label indicates the name and price of an article in characters and its code in a bar code. In a sales transaction, a key input operation to a cash register may be omitted by utilizing a bar code scanner. By causing labels affixed on articles to be sold to one customer to be successively faced to the bar code scanner by the cash register operator, the bar code scanner reads bar codes from the labels and inputs the article codes represented by the bar codes into the cash register. The cash register registers articles specified by the article codes fed by the bar code scanner and figures out the sum total of the registered articles.
A thermal printer which has been in use for issuing labels of the type mentioned includes a thermal printing head comprising of one line of dot-print elements and issues a label by printing, as shown in FIGS. 1A or 1B, characters and a bar code on a label sheet which is fed in a direction tangent to the direction of the print-elements line. As is well known, a bar code is made up of a combination of element bars, i.e., thin bars and thick bars and must be printed out in a manner meeting the standards on the bar code, which regulate as minimum requirements the length of an element bar, a minimum permissible value of the width of the element bar, and a ratio between the widths of the thick and thin bars.
Where a bar code is printed as shown in FIG. 1A, the widths of the element bars are dependent on the dot-print density of the printing head, i.e., the pitch of the print elements. For this reason, the print elements of the printing head need to be arrayed with a pitch that satisfies the requirements of the standards applicable to the type of bar codes to be printed. Assuming that an appropriate printing head is selected, printing in the format shown in FIG. 1B must be considered. In the FIG. 1B situation, the widths of the element bars are dependent upon the length of a label sheet that is moved forward during a sequence of printing head drive cycles, in other words, a rate of feed of the sheet. The above standards must be satisfied by the same conditions as in the case of FIG. 1A, so that the paper feed mechanism is designed to feed the sheet by the length equal to the pitch of the print elements of the printing head selected each time the printing head is driven.
A variety of bar codes are used worldwide today, examples of which include JAN, UPC, CODE39 and NW7. The bar code standards applicable to these bar codes have much to differ notably in the minimum permissible value of the width of the element bars and a ratio between the widths of the thick and thin bars. Where the type of bar code used is desired to be changed in the thermal printer described above, it has been necessary to replace the printing head and in addition, to re-design the paper feed unit. In particular, designing of the paper feed unit is not an easy task and this has resulted in an increase of cost in the manufacture of thermal printers.